FIG. 4.

Basal ERK1/2 phosphorylation and activity are defective in HIF-1α-null mEFs exposed to hypoxia or anoxia. ERK1/2 is not inducible in wt mEFs. (Top and center) Immunoblots of total protein from wt and HIF-1α-null mEFs harvested under normoxic conditions (air-5% CO₂) or following exposure to hypoxia (pO₂, ≤0.01%; 8 h). Replicate blots were probed with an anti-phospho-ERK1/2 antibody (specific for phospho-Thr202 and phospho-Tyr204 in the TEY activation motif) (top) or with an antibody that recognizes total ERK1/2 (center). (Bottom) Autoradiograph showing phosphorylation of myelin basic protein (MBP) by ERK1/2 immunoprecipitated from identical whole-cell lysates of wt and HIF-1α-null mEFs harvested under normoxic conditions (5% air-CO₂) or following exposure to hypoxia (pO₂, ≤0.01%; 8 h). PD 98059 (PD) was used as a control for ERK1/2 activity in these experiments. PD (final concentration, 50 μM) was added just before initiation of hypoxia.